Binding of an inducer to the lac repressor

The binding of an inducer, isopropyl-β- d-thiogalactoside, to the purified lac repressor in Escherichia coli was studied quantitatively by equilibrium dialysis. Although the repressor is composed of four identical subunits, the maximum number of inducer molecules bound per repressor molecule ( r max) was not four in general. It ranged from 2.3 to 4.0 depending on the conditions of assay, including the temperature and pH. Moreover the mode of binding of inducer varied with the conditions, exhibiting no co-operativity, positive co-operativity or negative co-operativity. It is suggested that in cells, where the pH is believed to be 7.2 to 7.4, the binding of an inducer to the free repressor is not co-operative and that the r max is about 3.4 at 37 °C. On the other hand, in the presence of an anti-inducer, o-nitrophenyl-β- d-fucoside, the binding of inducer showed an apparent positive co-operativity, in conditions where no co-operativity was observed in its absence, suggesting that the binding of an inducer is positively co-operative in the presence of the operator DNA.